1. Field of the Invention
The present invention concerns a method for reciprocal adaptation of a number of microphones of a hearing device. The present invention also concerns a corresponding circuit to adapt the microphones.
2. Description of the Prior Art
Hearing impaired persons frequently suffer a reduced communication capability in the presence interfering noise. To improve the signal-to-noise ratio, directional microphone arrangements have been used for some time, the benefit of which is indisputable for hearing impaired persons. Frequently, either systems of the first order (meaning with two microphones) or of a higher order are used. The exclusion of noise signals received from behind the person, as well as focusing on frontally incident sounds, enables a better comprehension in everyday situations.
Directional microphones, however, are sensitive with regard to detunings of the transfer functions of the microphones according to magnitude and phase. The sensitivity to detuning increases with the order of the directional microphone system and with decreasing frequency. Such directional microphone systems are most sensitive to detuning at low frequencies.
In this context, European Application 0982971 discloses that a microphone can be described or characterized at low frequencies as a high-pass filter of the first order. As shown in FIG. 1 herein, a first microphone 1 can be characterized as a high-pass filter with the transfer function a/s-pol_ac1. The microphone 1 acquires a first input signal 2. This input signal 2, filtered with the high-pass filter effect of the microphone 1, is transduced into a first microphone output signal 4 with of a first compensation filter 3. The compensation filter 3 has the transfer function s-pol_ac1/s-pol_ideal. Both numerator and denominator can be represented as polynomials. The numerator polynomial of the compensation filter 3 is selected such that it corresponds to the denominator polynomial of the acoustic high-pass filter characteristic of the microphone 1. The denominator polynomial of the compensation filter 3 corresponds to the denominator polynomial of the high-pass filter characteristic of an ideal microphone. By multiplying both transfer functions of the high-pass filter characteristic (that characterizes the real microphone 1) and of the compensation filter 3, a normalization results with regard to the ideal microphone and the specific transfer function of the first microphone is compensated.
For hearing device microphones, in a simplified approach, in particular the acoustic high-pass effect at the lower edge of the usable frequency band must be examined with regard to detunings. Contaminations, aging or modified environmental influences particularly strongly affect this region of the high-pass effect and thus modify the amplitude and frequency response of the microphone in the particularly critical middle and lower frequency ranges. A possibility to reduce such detunings is to enforce the same high-pass cut-off frequency in all microphone paths.
In the same manner, the specific high-pass effect is compensated with the transfer function s/s-pol_ac2 of the second microphone 5 with a second compensation filter 6 having the transfer function s-pol_ac2/s-pol_ideal, such that a corresponding second microphone output signal 8 arises from the second microphone input signal 7. Here the denominator polynomial of the high-pass filter 5 is also eliminated via the numerator polynomial of the second compensation filter 6. With both of these compensation filters 3 and 6, the variations of the high-pass frequency from microphone-to-microphone (that in particular would lead to phase and amplitude errors at low frequencies) can be compensated, by setting the same cut-off frequencies in all microphone paths.
A method for relative, adaptive phase compensation by two microphones is generally designed in U.S. Pat. No. 6,272,229. A general block diagram for an adaptive system is thereby specified. The system has a block “acoustical delay compensation” that, in a type of pre-processing, compensates the linear phase difference of the microphone that is a consequence of the signal delay between the microphones. No adaptation rule, however, is specified.
Further internal circuitry act primarily on the input sensitivity difference of the microphones. Conclusions or inferences about the input sensitivity of the microphones can be drawn via a temporally averaged consideration of the input level at the microphones. Assuming that the incoming audio signals are received time-delayed but with approximately the same level by all microphones, the amplitude of the input sensitivities can be compensated by a compensation of the averaged input level at the microphones.